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creased nor diminished during its con* 

 traction, what is lost in length being 

 gained in bulk. The voluntary contrac- 

 tion of muscles cannot be long continued ; 

 they become weary and painful, the con- 

 traction remits and recurs, causing a tre- 

 mulous motion. Yet this phenomenon 

 does not seem to be the effect of abso- 

 lute inability, in the irritable property, 

 to continue in action, for some muscles 

 continufe to act without experiencing fa- 

 tigue. For instance, those of the jaws 

 and back; for whenever they relax, the 

 jaw dropsy and the head and body fall 

 forwards, as we see in persons who are 

 going to sleep in a sitting posture. Certain 

 sphincter muscles likewise remain in action 

 without experiencing fatigue. Some sphinc- 

 ters also, I may add, are disposed to yield 

 considerably without impatience; so that 

 their irritability resembles that of those 

 muscles which Bichat has considered as- 



